As America braces for the 150th anniversary of the civil war, the California-based Not for Sale Campaign claims "there are more than 30 million slaves in the world today – more than at the height of the trans-Atlantic slave trade."

Yesterday, while the International Day for the Abolition of Slavery passed with little official fanfare, anti-slavery activists are busy tinkering with new media tools to catch the public's attention, using iPhones and "open-source activism" to try and reinvigorate the centuries-old humanitarian impulse to end slavery.

This November, the Not for Sale Campaign and the International Labour Rights Forum launched a new iPhone app, Free2Work, to help consumers reduce their "slavery footprint" by delivering product ratings as they shop.

The mobile phone application lets you browse and search for companies and products –from Lego to Levi's, and from food to footwear – each of which is awarded a grade from A to F.

An "A" grade is awarded to brands with sustainable and participatory systems to prevent forced labour, and a commitment to improve their industry as a whole. A "C" is the "standard" minimum passing grade, while an "F" goes to brands which are most at risk of using forced labour, as they do little or nothing to monitor their supply chain.

For those behind the Free2Work project, the hope is that the mobile phone application will help consumers deal with the complicated webs of supply-chains, outsourcing and subcontracting, and the plethora of monitoring, certification, and labelling programs that make it difficult for the average person to find an answer to the question: "Is my product slave-free?"

"The complexities of the global slave trade and limited insight into product supply chains make it difficult for the average consumer to grasp how he or she is connected to forced and child labour occurring within the global production cycle," says the Free2Work project.

The hope is that the mobile phone application will revolutionise how consumers make informed purchases by providing information when they need it most – while they shop.

"When companies know consumers have a tool to check their human rights and labour records at the point of purchase, they might just take those issues a lot more seriously," said Amanda Kloer an editor at Change.org, on her blog there.

Kloer has high hopes for how new media can invigorate contemporary anti-slavery campaigning, suggesting that Twitter can also be a useful tool.

"In 2010, we have a powerful weapon in the fight against slavery. Twitter. Yes, the 144-character social media tool might just do what all the legislation and international conventions have failed to — finally end slavery," she blogged.

But while the Free2Work app could attract much of the same criticism directed towards other campaigns for ethical consumerism, and while the suggestion that Twitter can end slavery is wide open for ridicule a la Malcolm Gladwell, historians and campaigners have often made the case that the British anti-slavery movement of the 18th and 19th centuries – which played no small part in the abolition of the slave trade – owed much of its success to its' activists innovative use of "new media."

In one example, Hochschild recounts the story of famous, one-legged pottery entrepreneur, Josiah Wedgwood, who had hundreds of medallions crafted featuring a bas-relief of a chained, kneeling slave, encircled by the words: "Am I not a Man and a Brother?"

"The equivalent of the lapel buttons we wear for an electoral campaign, this was probably the first widespread use of a logo designed for a political cause. It was the 18th century's 'new media,'" he writes.

Along with the medallions, Hochschild lists consumer boycotts, petitions, political posters, newsletters, direct-mail fundraising letters, political book tours, and national campaigns with local committees among the countless innovative tactics pioneered by the activists, and used ever since.

The Not for Sale Campaign say the major obstacle in the fight against modern-day slavery is that the crime is largely hidden, and that slavery is not part of the current collective consciousness. All too often, the end of the trans-Atlantic slave trade is interpreted as the end of slavery.

And while alerts of the "re-emergence" of slavery and calls to "re-abolish" it give campaigns and media reports a sense of much-needed urgency, they run the risk of misreading history, which has seen slavery disappear more from our public discourse than from our economy.

The widespread use of forced, indentured, and slave labour in Europe's African colonies, for example, is well-documented. And in his 2010 message for the International Day for the Abolition of Slavery, UN secretary general Ban Ki Moon stresses that "the abolition of the trans-Atlantic slave trade in the 19th century did not eradicate the practice globally."

"Instead, it took on other forms, which persist to this day: serfdom, debt-bondage and forced and bonded labour; trafficking in women and children, domestic slavery and forced prostitution, including of children; sexual slavery, forced marriage and the sale of wives; child labour and child servitude, among others," he adds.

Indeed, a key challenge has been defining what constitutes slavery in the 20th century and providing estimates of its scope.

It would, of course, be hard to imagine an iPhone app ending modern-day slavery. But as Hochschild pointed out in his Mother Jones article, the 18th and 19th-century sugar boycotts caught people's imaginations by bringing to light the hidden ties of an otherwise largely obscure globalised economy.

The moral outrage that followed produced the first mass human rights campaign, and played a significant part in the end of the trans-Atlantic slave trade. Could the Free2Work app – and future new media innovations, still to come – do the same for us today?